This invention relates to weldable, alloy steels having high ultimate tensile and yield strength in combination with both high stress corrosion resistance and high toughness which together make it desirable for aerospace vehicular and other fracture critical structures. Design requirements for structural metallic materials used in airplane or like usage include a high strength to weight ratio, high stress corrosion resistance, high fracture or notch toughness, and ease of fabrication. A stress corrosion resistance to fracture toughness (K.sub.I.sbsb.SCC /K.sub.I.sbsb.C) ratio greater than 0.5 is highly desirable for aircraft structural components as well as any application where the maximum operating load is two or less times the steady state sustained load. Such a ratio insures that no stress corrosion cracking will occur during sustained load operation if the structure is designed to resist brittle fracture at maximum operating load. (K.sub.I.sbsb.SCC and K.sub.I.sbsb.C are the stress intensities (Ksi .sqroot.inch) below which stress corrosion cracking will not occur within 1000 hours in 3.5% NaCl and brittle fracture will not occur, respectively).
As referred to herein, fracture resistance is measured in terms of notch toughness (CVN), a measure of resistance to fracture under impact loading in ft-lbs in presence of a notch, and fracture toughness (K.sub.I.sbsb.C), which is resistance to fracture under loading in presence of a crack. In the steels of the present invention fracture toughness measured as Charpy V-notch (CVN) can be closely correlated empirically with the measurement obtained by the fracture mechanics test for K.sub.I.sbsb.C. Fracture resistance is also a function of stress corrosion resistance -- (K.sub.I.sbsb.SCC) which measures resistance to crack growth in a corrosive environment under sustained load in the presence of a crack.
The art is replete with steels which have been developed for broad spectrum usage as well as for special needs including the needs of the aerospace industry. Many of the prior steels used for aerospace applications, e.g., HY-180, 300M, D6ac, maraging steels and others, provide various combinations of strength, fracture toughness and stress corrosion resistance. Some may also be welded. For example, U.S. Pat. No. 3,502,462 to Dabkowski et al for Nickel, Cobalt, Chromium Steel discloses steels in the range of up to about 197 Ksi maximum yield strength (tensile) having excellent toughness and stress corrosion resistance. There has been a need, however, particularly in the aerospace field for a steel which is at once weldable and provides the best combination of low weight with good stress corrosion resistance and toughness at higher strength levels than heretofore available, particularly up to about 270 Ksi ultimate strength (TUS) or about 245 Ksi yield strength (TYS). Good fatigue endurance limits are also required. Steel groups which are currently available for service at these strength levels are the low alloy medium carbon quench and temper steels, maraging steels, and high strength stainless steels. Also, high toughness (fracture toughness or notch toughness) at high strength levels does not necessarily indicate that a high stress corrosion resistance will be obtained.
The low alloy medium carbon steels in this strength range require carbon contents in excess of 0.3% to meet strength requirements at the expense of fracture and stress corrosion resistance and weldability. The principal strengthening mechanism is the tempering of the carbon martensites which produce a precipitation of carbide particles generally detrimental to high toughness and stress corrosion resistance at this strength level. However, as carbon alone is increased there is an increased tendency for microcracking due to increased lattice strains present as a result of higher tetragonal distortion. This condition can be somewhat alleviated by adding substantial amounts of solid strengtheners, e.g., Ni., Cr., Co., Mn., which will reduce the level of carbon necessary to attain high strength. These alloys, although still categorized as quench and temper steels, and having improved toughness and stress corrosion resistance due to the alloy martensitic matrix yet are below the strength levels found desirable for those structures requiring the highest strength with improved toughness and stress corrosion resistance.
Maraging steels develop high strength as a result of complex precipitation reactions in a low carbon iron-nickel martensite formed above room temperature. Titanium and aluminum are added so that during aging the maximum strengthening will occur by formation of complex nickel-aluminum, nickel-titanium, and nickel-molybdenum intermetallic compounds in the high toughness martensite matrix. As a result more toughness is possible at higher strength levels than is attainable in ordinary quench and temper steels. However, such intermetallic compounds, which are used for strengthening, tend to reduce the stress corrosion resistance. Also, the necessary presence of titanium and aluminum in these steels require caution to keep the residual elements at low levels, these elements being strong carbide, nitride and oxide formers. Formation of an excess of these compounds is difficult to prevent in making of these steels and if present will result in substantial reductions in tougness. Further, since highly cored fusion zone structures are inherent to the maraging system the toughness of the weld deposit is usually always below that of the parent metal.
High strength stainless steels capable of obtaining ultimate tensile strength exceeding 220 Ksi and yield strengths above 210 Ksi are usually of the semiaustenitic or martensitic precipitation hardening type. In general all these alloys have high chromium contents necessary for good corrosion resistance, but as a group have low fracture toughness and stress corrosion resistance, particularly when heat treated to the maximum strength. Some of these steels also are not suited for use at cryogenic temperatures as fracture toughness may decrease appreciably. Although some of the steels are partially austenitic in the solution treated condition at room temperature, it is possible to complete the transformation to martensite by a series of thermal treatments or by cold working to gain increased strength. Some of the TRIP group of the stainless steels when subjected to thermomechnaical working may improve fracture toughness at high strength levels. However, the latter, unlike steels of the present invention, are limited by factors such as plate thickness size and other factors.
None of the referred to prior steels, however, provide the desired levels of fracture toughness and stress corrosion resistance at the high strengths achieved by steels as taught herein.